Since the first report on the efficient visible photoluminescence of porous silicon (PSi) under (ultra violet) UV excitation, at room temperature, this material has generated world wide interest. There has been a vast amount of work devoted to the structural, optical and electronic aspects of this material in order to understand the origins of the photoluminescence and to develop applications in solid-state electroluminescent devices. Because of its tunable electroluminescent and photoluminescent properties, applications in silicon-based optoelectronics, which have hitherto been severely constrained by the weak luminescence of indirect band-gap bulk silicon, have become feasible. The origin of the quantum confined photoluminescence is believed to be due to the Si nanocrystallites present in the porous layer. Several models have been proposed to explain the photoluminescence contributions from other species on the PSi surface. Potential applications based on electrical and/or optical measurements for sensing chemical and biochemical species have been demonstrated using PSi.
It is found, however, that a freshly-prepared PSi surface is covered with a monolayer of hydrogen (Si—Hx). The hydrogen-passivated PSi film is of good electronic quality, but the monolayer of hydrogen formed on its surface does not protect against photoluminescence quenching from chemical adsorbates leading to slow degradation of photoluminescence exposure to air and concomitant degradation of the electronic properties of the material. This limitation restricts the use of PSi in the fabrication of commercial devices. The hydrogen-terminated PSi surface reacts in ambient air to form an oxide sub-monolayer, which introduces the surface defects responsible for the photoluminescence quenching. Many efforts have been made to stabilize the H-terminated surface in order to protect the PSi from photoluminescence fatigue. Deliberate oxidation of the surface is one of the most studied reactions to achieve this goal, under thermal, electrochemical, or chemical conditions. Under controlled conditions, thermal oxidation provides good results in preserving a red surface-related photoluminescence and a light-emitting device based on thermally-oxidized PSi was recently reported. However, passivation of the PSi surface by oxidation restricts the photoluminescence to red wavelengths, and is not suitable for stabilizing the photoluminescence of high porosity PSi. For example, blue photoluminescent PSi tends to react quickly with oxygen upon exposure to ambient air and the photoluminescence shifts after only a few seconds to the red. Recently, it has been found that etching of silicon single crystals in a mixture of hydofluoric acid and ferric nitrate aqueous solution leads to a stable red photoluminescent PSi. This effective passivation is attributed to the presence of Si—Fe bonds on the PSi surface. However, the presence of metals such as iron on the semiconductor surface may severely limit the use of PSi in advanced semiconductor technology.
More recently, there has been increasing interest in the chemical modification of silicon surfaces. This strategy has been used successfully in the passivation of flat and porous silicon surfaces, and in the preparation of organic monolayers chemically stable in different organic and aqueous media. Advantages associated with these transformations include: the existence of a wide range of chemical functionalities compatible with the Si—H bonds terminating the PSi surface, the ease of carrying out the chemical reactions, and finally, the very well established organosilicon chemistry in solution. Scaling down to the molecular level will open new opportunities for a new generation of devices.
Both the formation of Si—O—C and Si—C linkages on PSi have been studied. Formation of organic monolayers containing Si—OR linkages has been achieved by photoelectrochemical reaction with carboxylic acids, and electrochemical and thermal reaction with alcohols of freshly prepared PSi surfaces. Stabilization of the PSi surface through Si—C linkages has been achieved by a direct reaction of hydrogen-terminated PSi surfaces with alkyl Grignards and lithium reagents under electrochemical and thermalconditions. In the latter case, the reaction occurs with Si—Si bond cleavage to give Si—C and Si—M (M=Mg, Li) bonds. The latter intermediate could be functionalized with different electrophiles. Hydrosilylation of hydrogen-terminated surfaces with alkenes and alkynes has been applied in the presence of a Lewis acid as a catalyst.
Such approaches are based on the substitution of H with more densely packed small molecules, such as oxygen, metals, and organic molecules. Oxygen stabilization affects the photoluminescence intensity, and the energy is fixed in the red. Metals have been found not to provide good coverage within the pores. Organic molecules have been found to provide the best results, but they still suffer from the fact that eventually oxidation occurs.
Bateman et al, Chem Int. Ed. 1998, 37, 2683-2685 described the application of organic molecules at elevated temperatures, but it is found that structures made by his method are partially oxidized, have poor chemical stability and poor luminescence properties.
All approaches to the problem of stabilization have so far been based on the substitution of H with more densely packed atoms or small molecules, such as oxygen, metals, and organic molecules. Oxygen stabilization affects the photoluminescence intensity, and the energy is fixed in the red. Metals have been found not to provide good coverage within the pores. Organic molecules have been found to provide the best results, but they still suffer from the fact that eventually oxidation occurs. There is, therefore, a need for an improved method of stabilization which does not suffer from the disadvantages of existing methods.